(1) Field of the Invention
The present invention relates to the fabrication of semiconductor circuits, and more specifically to a method for forming bonding pads such that bonding pad corrosion is eliminated for bonding pads whose top metal surface contain minute amounts of copper and silicon.
(2) Description of the Prior Art
For many of the advanced semiconductor devices, device signals such as ground, power and I/O signals require numerous bonding pads. With the increased density of components within a chip and with increased sophistication of the circuitry contained within the chip, further demands are placed on the number of bonding pads for each chip. For many designs, the number of bonding pads becomes the limiting factor on chip size and chip function.
Improvements in packing density cannot be realized by simply shrinking the design rules or adding more levels of metal wiring.
Aluminum grows a passivating oxide layer in air and is as a consequence protected against corrosion. Aluminum wiring used in semiconductors, however, contains copper, which does not have a passivating oxide, and the Al--Cu alloy used is more vulnerable to corrosion. The corrosion of aluminum wires is caused by several sources such as chlorine transported through the plastic packaging and the passivation materials, chlorine from the etching compounds and as etching by-products, phosphorous acid formed from excess phosphorous in the phosphosilicate glass, etc. Only a small amount of chlorine is required to cause severe local corrosion of the aluminum lines. Aluminum corrosion can, in addition, occur very quickly after metal etching.
To avoid etching introduced corrosion, chlorine compounds and elemental chlorine must be removed from the metal surface immediately after plasma etching. This is usually accomplished by a water rinse or a water vapor treatment.
Modern metal structures use multi-levels of dissimilar materials such as Ti/TiN/Al--Cu/TiN or Ti/Al--Cu/TiN which increases the possibility of electromechanical corrosion.
Copper is electro-positive with respect to hydrogen and is not vulnerable to corrosion. However, in air copper oxide grows linearly with time, indicating the lack of a protective oxide. This lack of a passivating oxide makes copper more vulnerable to chemical corrosion. To avoid or minimize this corrosion, most applications of copper metalization involve some protective layer deposited on top of the copper.
FIG. 1 shows a cross section of a typical Prior Art bond pad. Bonding pads serve the function of providing device signals such as ground, power and I/O signals to semiconductor devices. In this capacity a bonding pad provides an electrical path between the semiconductor device and the electrical circuitry and components with which the semiconductor device interfaces. The bonding pad structure provides for an opening to which one contact point of the semiconductor devices is connected, from this bonding pad opening the electrical path continues through the bonding pad to a metal plate that connects with the electrical circuitry and components with which the semiconductor device interfaces. Where this connecting electrical path consists of Al--Cu, top metal surface corrosion and pitting is a problem within the opening of the bonding pad.
Referring now specifically to FIG. 1, there is shown a cross section of a typical Prior Art bonding pad. The metal layer 10 connects the bonding pad with the electrical circuitry and components with which the semiconductor device interfaces, the semiconductor device is connected to the bonding pad via the opening 18. A layer 14 of Al--Cu provides the electrical path between metal 10 and opening 18. A layer 26 of dielectric is deposited over the metal layer 10, a via hole 12 is etched in this dielectric layer 26. A passivation layer 24 is deposited on top of the Al--Cu layer 14, the opening 18 is etched into this passivation layer 24. The passivation layer 24 is separated from the metal layer 14 by a layer 16 of anti-reflective coating (ARC). Surface pitting 20 and surface corrosion 22 occur on the surface of top metal layer 14 within the opening 18 of the bonding pad.
FIG. 2 shows the processing steps required to create a typical Prior Art bonding pad. The numbers indicated within the following description of the processing steps required to created a Prior Art bonding pad refer to the numbers that have previously been used under FIG. 1. The bonding pad is constructed starting with the original metal 10 (FIG. 2, step 1). On top of this metal 10 a layer 26 of dielectric is deposited (step 2). Via 12 is created within the dielectric 26 (step 3), a layer 14 of Al--Cu is deposited across the dielectric 26 and the via 12 (step 4). A layer 16 of ARC is deposited on top of the layer 14 of Al--Cu (step 5). A passivation layer 24 is deposited over the ARC layer 16 (step 6). The bonding pad opening 18 is etched within the passivation layer 24 (step 7).
U.S. Pat. No. 5,661,081 (Hsue et al.) teaches an Al or Al alloy bonding pad. However, this reference differs from the invention.
U.S. Pat. No. 5,731,243 (Peng et al.) shows a method to reduce residue formation over an Al bond pad.
U.S. Pat. No. 5,559,056 (Weiler) shows a bond layer over a bonding pad.
U.S. Pat. No. 5,525,546 (Harada et al.) shows an Al alloy bonding pad and method.
U.S. Pat. No. 5,565,378 (Harada et al.) shows a method of passivating an Al bonding pad using an O.sub.3 solution.